Performance venues such as theaters, arenas, concert halls, auditoriums, convention centers, television studios, and the like can employ battens or trusses to raise and lower lighting, scenery, set-pieces, displays, draperies, and other items. Lift assemblies, or hoists, are typically used to raise and lower battens or trusses and attached loads. The lift assemblies are commonly connected directly to the support structure of a building, for example, to overhead beams. In some lift systems, multiple lift assemblies, or machines, can be employed for moving heavy loads, and can be connected to the same support structure.
Variable numbers of lift machines can be selected to operate for moving particular loads, such as a stage curtain and scenery. In a situation in which the stage curtain and scenery need to be raised at the same time, two lift machines can be selected to operate simultaneously. When multiple lift assembly machines are started, stopped, sped up, or slowed down at the same time, the moving mass and inertia of the machines and attached loads can place a large dynamic load on the support structure. As used herein, “dynamic load” refers to a dynamic mechanical load created by the acceleration or deceleration of a mass. For example, dynamic load on a building structure can be created by the force exerted by the inertia of starting, stopping, speeding up, or slowing down one or more accessories connected to the structure.
In locations where lift assemblies are installed, for example, to the “flytower” above a stage, the building support structure is often designed to handle the dynamic load of only a few lift machines starting and/or stopping at the same time. If too many lift machines are started and/or stopped at the same time, the associated dynamic load can cause damage to the support structure. Accordingly, the number of machines that are started or stopped at the same time may need to be limited in order to limit the dynamic load created.
Lift assembly systems that employ multiple lift machines often include a primary safety mechanism to prevent excessive dynamic loading on the support structure when the machines are started or stopped. Generally, such safety mechanisms are controlled through software. One risk of a software-based safety mechanism is that the software can malfunction or fail due to loss of power, inherent or acquired bugs, misuse by an operator, or other reasons. Thus, it is often desirable to have a dynamic load safety backup system that prevents the start of too many machines.
Some conventional multi-machine lift systems utilize an operator-activated safety backup mechanism to avoid overloading the building support structure to which a system is connected when multiple machines are started, stopped, or speed changed at the same time. For example, when signaled that an excessive dynamic load is being exerted by start-up of multiple machines, an operator can hit an “emergency stop” button to shut off power and stop operation of the machines. A significant disadvantage of such an operator-activated safety mechanism is that simultaneously stopping operation of multiple machines can suddenly release the excessive dynamic load in one direction and thereby create an excessive dynamic load on the support structure in the opposite direction. Another disadvantage is that such an operator-activated safety backup mechanism is engaged “after the fact,” following initiation of an excessive dynamic load, and is dependent upon an operator monitoring for an excessive load.
Some conventional multi-machine lift systems utilize a software-based program as a safety backup mechanism to avoid an excessive dynamic load. Such software allows only a limited number of machines to be selected for movement at one time. One disadvantage of a software-based safety mechanism is that the software can malfunction or fail due to bugs in the software, or when used in applications that exceed software parameters. Another disadvantage of such a software-based safety mechanism is that certifying such systems for safety according to regulatory and/or industry standards can be complicated (if not impossible), time-consuming, and costly.